Date Published: June 27, 2018
Publisher: Public Library of Science
Author(s): Wilson Mendoza, Dominick Mendola, Jang Kim, Charles Yarish, Alyssa Velloze, B. Greg Mitchell, Yiu Fai Tsang.
This work developed a laboratory prototype methodology for cost-effective, water-sparing drip-irrigation of seaweeds, as a model for larger-scale, on-land commercial units, which we envision as semi-automated, inexpensive polyethylene sheet-covered bow-framed greenhouses with sloping plastic covered floors, water-collecting sumps, and pumped recycling of culture media into overhead low-pressure drip emitters. Water droplets form on the continually wetted interior plastic surfaces of these types of greenhouses scattering incoming solar radiation to illuminate around and within the vertically-stacked culture platforms. Concentrated media formulations applied through foliar application optimize nutrient uptake by the seaweeds to improve growth and protein content of the cultured biomass. An additional attribute is that seaweed growth can be accelerated by addition of anthropogenic CO2-containing industrial flue gases piped into the head-space of the greenhouse to reuse and recycle CO2 into useful algal biomass. This demonstration tested three different drip culture platform designs (horizontal, vertical and slanted) and four increasing fertilizer media concentrations (in seawater) for growth, areal productivity, and thallus protein content of wild-collected Ulva compressa biomass, against fully-submerged controls. Cool White fluorescent lights provided 150–200 μmol photon m-2 s-1 illumination on a 12/12 hr day/night cycle. Interactive effects we tested using a four-level single factorial randomized block framework (p<0.05). Growth rates and biomass of the drip irrigation designs were 3–9% day-1 and 5–18 g m-2 day-1 (d.w.) respectively, whereas the fully-submerged control group grew better at 8–11% per day with of 20–30 g m-2 day-1, indicating further optimization of the drip irrigation methodology is needed to improve growth and biomass production. Results demonstrated that protein content of Ulva biomass grown using the vertically-oriented drip culture platform and 2x fertilizer concentrations (42:16:36 N:P:K) was 27% d.w., approximating the similarly-fertilized control group. The drip methodology was found to significantly improve gas and nutrient mass transfer through the seaweed thalli, and overall, the labor- and-energy-saving methodology would use a calculated 20% of the seawater required for conventional on-land tank-based tumble culture.
Reports by The Food and Agriculture Organization of the UN (FAO) project a doubling in demand for seafood in the 21st century, with most of the increased demand to be met by aquaculture [1–5]. In Asia and other parts of the world, seaweed has been a cost-effective food commodity cultured using a traditional long-line method in coastal seas [1, 6]. However, much of the future demand for aquacultured products will be for protein-rich fish and/or shrimps that currently are fed almost exclusively on fishmeal-containing manufactured aquafeeds [7, 8], which is viewed as an environmentally, ecologically and socially unsustainable practice [4, 9,10].
Three drip irrigation platform designs were evaluated and tested in triplicate. The first was a Multi-Level Horizontal Design (MLHD), three, identical, vertically-stacked and tightly coupled square plastic containers (8” x 8” base x 3” height) were used. The uppermost container served as a media holding reservoir to receive input media via a 4-mm silicone rubber tube attached to a small, submerged aquarium-type pump located in the bottom-most container. The bottom of the upper reservoir was totally drilled with an array of equally-spaced 2 mm-diameter holes on a 5 x 5 mm spacing pattern, and thus served as the drip irrigation provider. The seaweed biomass for testing was contained in the middle container which had a similar hole-drilled bottom covered by a square fine-mesh nylon netting to prevent clogging of the holes by small pieces of seaweed. To commence an experiment, a pre-weighed mass of Ulva seaweed thalli was placed into the middle container and the three containers tightly coupled together. The pump was switched on to commence the recirculation of the culture media at a plastic valve-set rate flow rate (approx. 1 ml s-1), and the date and time were recorded to begin the experiment.
Comparing the mean daily dry biomass productivity from the three drip irrigation platforms to the submerged control platform, showed that the submerged control platform consistently maintained the highest biomass productivity (range = from 20–30 g m-2 d-1), whereas biomass productivity for the three drip-irrigated platforms ranged from 5–18 g m-2 d-1, (Fig 2).
Our results showed that both the sloped (SD) drip culture platform design, and the multi-level horizontal design (MLHD) produced significantly higher rates of NO3 uptake when compared to the totally submerged controls (SUB). Additionally, the bag-pouch vertical drip culture platform design (BPVD) produced the highest uptake rates of NH4+ and PO4-3 when compared to the SUB controls. These results support a physiological adaptation mechanism in the drip irrigated groups expressed as increased rates of liquid and gas mass transfer through relatively thinned cellular diffusive boundary layers mediated by the continual and repeated “pelting” of steady streams of nutrient-rich media falling onto and thence off of the thalli surfaces. This mechanism does not apply for the totally submerged controls where water movement past the seaweed thalli is much, much reduced compared to the fast-paced drip irrigation.